Because of the enormous growth of computerization in the office environment, especially in the area of personal computers and their associated peripheral devices such as keyboards, printers etc., there is an increased need for flexible interconnection of these devices. Wireless infrared links are an attractive alternative to cabling. Such communication systems can be used beyond the office environment, i.e. anywhere communication is required between intelligent devices so long as sufficient signal power is available for the receiver.
Infrared free space links, where diffusely scattered infrared radiation serves as a carrier for the transmitted data, have numerous advantages. They allow greater flexibility in the placement of equipment within a room and easy reconfigurations. Since the infrared radiation is essentially confined to the room in which it is generated and cannot be easily detected from outside the room, there is no interference between transmission links operating in adjacent rooms. Freedom from electromagnetic interference is another significant advantage. The cost reduction in comparison with cabling constitutes another important advantage of infrared free space links over the use of cabling. Most of the data transmission systems using infrared links allow communication at 1 MBit/S over 10 to 15 meters by using diffuse reflections from walls and ceilings with several hundred milliwatts of optical power and a receiving photodiode area of 1 square centimeter.
Use of fluorescent activated optical detectors is known from U.S. Pat. No. 4,371,897 to Kramer. However this patent relates to a spatial, quantitative light detector for detecting light scattered from an information-bearing surface and is not concerned with increasing effective transmission distance in infrared free space data communication systems
U.S. Pat. No. 4,506,153 to Ohno relates to an apparatus for transmitting light pulses of two different wave lengths along optical fiber paths to ON-OFF switches to detect an ON or OFF state of the switches by reflection/non-reflection dependent on switch status.
To widen applicability of free space infrared links in data communication systems it is desirable to increase the effective transmission distance between communicating devices without increasing or even in the face of decreasing optical transmission power. One way of achieving such an increase in transmission distance is by increasing the detector sensitive area so as to collect more infrared signal energy. However, this conventionally involves the use of a large area photodiode which is extremely expensive. Furthermore such diodes have capacitance of more than a hundred pF which would require an expensive receiver (for adaptive equalization and/or very high gain band-width product if a transimpedance configuration is chosen). As a practical matter, it is not feasible to increase the effective area for diffuse infrared radiation with passive optical systems such as lenses or mirrors.
The above described difficulties concerning the increase in transmission distance for data communication systems are solved by the present invention through use of a fluorescent activated infrared light detector coupled with optical transmission systems utilizing bipolar signals.